1. Field of the Invention
The present invention relates to a jitter detecting apparatus for detecting and compensating for jitter in a VTR recording/reproducing video signal applied to a recording/reproducing apparatus which records and reproduces a video signal by multiplying a luminance signal and a chrominance signal on time bases.
2. Description of Related Art
In a video signal reproduced by a recording/reproduction apparatus, a time base inevitably fluctuates and jitter is often generated. Jitter includes a phase jitter and a frequency jitter (velocity error). In principle, the frequency jitter causes the expansion and compression of a horizontal period to occur.
Normally, the fluctuation amount of the frequency jitter is very slight even on a reproduced picture from a home video system having a relatively simple mechanism. Therefore, the correction of the phase jitter of a reproduced video signal is enough to ensure the stabilization of a reproduced image.
The correction of the phase jitter generated in a video signal can be useful in the following processes without resulting in problems: The decrease of random noise using a frame correlation, the synthesizing of two-channel video signals, specific processings such as a fade-in or a fade-out, and a digital processing operation for performing double speed scanning in high density television (for example, HDTV).
FIG. 11 shows an example of a jitter detecting circuit 20, for correcting a phase jitter, for use in such a recording/reproducing apparatus (hereinafter referred to as VTR.)
The jitter detecting circuit 20 shown in FIG. 11 is used for a broadcasting VTR in which composite video signals generated by an NTSC system, PAL system, or the like are recorded by a direct frequency modulation recording system.
A reproduced video signal (a) (shown by (A) in FIG. 12) of, for example, the NTSC system which is supplied to a terminal 1 is inputted to a synchronizing pulse separating circuit 2 in which a horizontal synchronizing signal (b) (shown by (B) in FIG. 12) is extracted and separated from the video signal (a). A burst gate pulse (not shown) generated from the horizontal synchronizing signal (b) is supplied to a burst gate circuit 3 in which a burst signal (c) (shown by (C) in FIG. 12) is separated from the input video signal.
The burst signal (c) is supplied to a narrow bandpass filter 4 in which a burst signal (d) (shown by (D) in FIG. 12) high in C/N ratio is produced.
The horizontal synchronizing signal (b) is supplied to a delay pulse generating circuit 5 in which a delay pulse (e) (shown by (E) in FIG. 12) is generated. The delay pulse (e) and the burst signal (d) which has passed through the narrow bandpass filter 4 are supplied to a jitter pulse generating circuit 6, thus a jitter detecting pulse (f) (shown by (F) in FIG. 12) being generated. The phase of the leading edge of the jitter detecting pulse (f) lags by a predetermined period of time behind the phase of the horizontal synchronizing signal (b).
The phase of the trailing edge of the jitter detecting pulse (f) is obtained by detecting a specific zero crossing point of the burst signal (d). The write timing of a time base correcting circuit (TBC) is determined on the basis of the timing of the trailing edge of the jitter detecting pulse (f).
The fluctuation of the time base in the reproduced video signal (a) causes the time base of the specific zero crossing point of the burst signal (d) to fluctuate. Thus, a video signal can be written in the time base correcting circuit (TBC) in synchronization with a generated jitter.
A burst signal is not produced if a TCI (Time Compressed Integration) system is adopted as a video signal recording system whereas the burst signal is generated in the direct frequency modulation recording.
FIG. 13 shows the signal format of MTCI (Modified Time Compressed Integration) system, of the TCI system, in which a band-compressed chrominance signal is recorded in a line sequential system.
Compressed component chrominance signals, for example, color-difference signals R-Y of red and B-Y of blue are inserted and multiplied in a horizontal blanking period as shown by (A) and (B) in FIG. 13.
Reference symbol (Y) denotes a luminance signal. The color-difference signals R-Y of red and B-Y of blue are inserted into the line sequential system.
Normally, a jitter detecting signal is obtained by detecting the rising or falling portion of a horizontal synchronizing signal in a video signal reproduced by such a signal format.
As described above, a jitter detecting signal is detected according to the detection of the rising or falling portion of the horizontal signal in the TCI recording system. This system has a disadvantage in that jitter detection cannot be performed with high accuracy.
The generation of a jitter detecting signal based on the falling portion of the horizontal synchronizing signal is described with reference to FIG. 14.
Normally, noise (N) whose phase and level are random is superposed on a horizontal synchronizing signal. Therefore, if a detecting level is set to (A), the detection timing fluctuates by .sub..DELTA. T caused by the phase of the superposed noise (N). The fluctuation amount .sub..DELTA. T (fluctuation amount of the time base, namely, amount of synchronizing jitter) affects the accuracy in performing a jitter detection.
The fluctuation amount .sub..DELTA. T of a horizontal synchronizing signal is approximated at an intermediate level assuming that its signal band is 4 MHz and the S/N is 40 dB.
As shown in FIG. 15, assuming that the amplitude of a horizontal synchronizing signal is 1 Vp-p, the effective value of the noise (N) superposed on the horizontal synchronizing signal (b), whose amplitude is 0.3V, is approximately 10 mV. Supposing that the falling portion of the waveform of the horizontal synchronizing signal can be approximated by 1/2 cycle of a sine wave of 4 MHz, the inclination (K) of the level at the center of the sine wave is expressed as follows: EQU K=125 ns/ (0.3 (.pi./2)V) =265 ns/V
Assuming that the peak-to-peak value of the superposed noise (N) is six times as great as that of the effective value, the peak-to-peak value of the fluctuation amount .sub..DELTA. T is expressed as follows: EQU .sub..DELTA. Tp-p=265.times.0.01.times.6=16 ns
That is, jitter caused by the noise (N), accurately detected cannot be below 16 nsec.
In addition, as shown in FIG. 14, a detecting level may fluctuate from (A) to (B). The fluctuation of the detecting level causes the jitter detecting accuracy to fluctuate to a great extent. In order to limit the fluctuation amount .sub..DELTA. T' to below 20 nsec, it is necessary that the fluctuation amount of a detecting level does not exceed 50 mV, which results in a great increase of the cost of a detecting level generating circuit. In this respect, such a method for generating a jitter detecting signal is not preferable.
The fluctuation of a detecting level occurs also due to the fluctuation of a clamp level and the amplitude fluctuation of a video signal.
The above approximated value of the fluctuation amount is obtained from a VTR having an accurately constructed circuit. However, the fluctuation amount .DELTA.T may be as great as 100 nsec in home VTR systems.
Thus, the generation of a jitter detecting signal in accordance with the rising or falling portion of a horizontal synchronizing signal gives rise to the problem that jitter cannot be detected with high accuracy.
As a means for solving this problem, the following method is considered: The phase of a frequency modulation carrier is controlled so that it becomes 0.degree. or 180.degree. for every horizontal period. However, the following problem arises if this method is carried out.
That is, normally, as shown by (B) in FIG. 17, the phase of the frequency modulation carrier is discontinuous before and after a point at which the phase of the frequency modulation carrier is reset.
When a signal recorded with the phase of the frequency modulation carrier being discontinuous is reproduced, as shown by (C) in FIG. 17, a level change (defect) occurs in the reproduced video signal in the shape of a sawtooth at the point corresponding to the discontinuity of the frequency modulation carrier phase.
Thus, such a fluctuation of the frequency-demodulated output causes a large amount of demodulation error at this point.
The clamp system ,of a reproduced video signal employed in a home VTR system is a sync chip clamp system. Therefore, a defect of the sync chip may cause a clamp error, which causes a defective detection of a synchronizing signal and a change of the luminance level of the reproduced output signal.